At the present time, it is possible to commercialize an optical disc having a recording density of approximately 100 GB for the people's livelihood as well owing to Blu-ray Disc™ standards using a blue-violet colored semiconductor laser. In the future, a large capacity exceeding 500 GB is desired in optical discs as well. For implementing such an ultra-high density in optical discs, however, a density increasing technique using a new system different from the conventional density increasing technique using a shorter wavelength and a high NA is needed.
Among studies conducted concerning storage techniques, the hologram recording technique for recording digital information by utilizing holography attracts attention. As for the hologram recording technique, for example, there is JP-A-2004-272268 (Patent Literature 1). The so-called angle multiplex recording system, in which different page data are displayed on a spatial light modulator while changing an incidence angle of a reference beam to an optical information recording medium and multiplex recording is conducted, is described in JP-A-2004-272268. In addition, a technique of shortening the spacing between adjacent holograms by condensing a signal beam with a lens and disposing an aperture (spatial filter) in its beam waist is described in JP-A-2004-272268. Furthermore, as for the hologram recording technique, there is, for example, WO2004-102542 (Patent Literature 2). An example using a shift multiplex system is described in WO2004-102542. In one spatial light modulator, a beam from inner side pixels is used as a signal beam and a beam from outer side ring-shaped pixels is used as a reference beam. Both beams are condensed onto an optical information recording medium by using the same lens. A hologram is recorded by causing interference between the signal beam and the reference beam near a focus plane of the lens.
Furthermore, for example, in JP-A-6-84762 (Patent Literature 3), there is the following description. “At the time of hologram recording, a monitor beam 40 differing in wavelength from a reference beam 18 and an object beam 28 is incident on a prism 16 obliquely. A hologram recording medium 17 is irradiated with the monitor beam 40. A part of the monitor beam 40 reflected by an inner face of the hologram recording medium 17 is diffracted by a formed hologram. The monitor beam 40 that is not diffracted is transmitted by the hologram recording medium 17, emitted from an oblique plane on the opposite side of the prism 16, and detected by a detector 31. The recording state of the hologram is detected by detecting at least one of the diffracted beam and the non-diffracted beam of the monitor beam 40. The exposure condition can be optimized on the basis of the detected recording state of the hologram.”